The present invention relates to a process for stabilising particulate alkali metal percarbonate, and particularly to a coating process therefor, to the particulate percarbonate having improved stability so produced and to washing or bleaching compositions containing the same.
It is well known that alkali metal percarbonates, including sodium percarbonate in particular, can be used as bleaching compounds in detergent powder mixtures, e.g. for domestic clothes washing. In comparison with alkali metal perborate tetrahydrate, they have the advantage of dissolving more rapidly at 20.degree. C., which is becoming increasily beneficial in view of the trend towards lower washing temperatures. Another advantage of percarbonates is that they are environmentally friendly. However, percarbonates have a known disadvantage of decomposing more quickly than sodium perborate tetrahydrate during storage in the powdered state, particularly if stored in a damp atmosphere. Moreover, other constituents of washing compositions can accelerate their decomposition.
In order to improve the stability of percarbonates, and particularly sodium percarbonate, it has been proposed to bring the percarbonate particles into contact with various stabilising materials, and in particular to coat the percarbonate with those materials. Organic and/or polymer compounds such as paraffins, polyols, vinyl resins etc. and inorganic compounds such as silicates, borates, perborates, boric acids etc. have been proposed as coating materials.
A French patent to Kao Corporation published under number 2 528 447 describes coating the surface of sodium percarbonate with sodium borate. A process for coating sodium percarbonate consists of wetting the sodium percarbonate with water, mixing the damp sodium percarbonate with sodium borate in powder form and then drying them at a temperature above the melting temperature of the sodium borate used. This process requires a substantial amount of energy because of the high temperature required to melt sodium borates. In addition, the fact that the sodium percarbonate is wetted requires strict control of its water content to prevent or minimise decomposition.
In British patent 1 575 792, to Interox SA there is described a process for stabilising sodium percarbonate by coating it with a solution of a boric acid. We have found that coatings with boric acid can be very effective at stabilising percarbonate, but the solubility of boric acids in water is somewhat limited, particularly at ambient temperatures. In consequence, it is necessary to use rather larger volumes of coating solution than for more soluble coating materials, which is disadvantageous because it would be desirable to find a way of increasing the solubility. Such low concentrations are disadvantageous for industrial use, because more thermal energy is needed to dry the percarbonate particles after coating, thereby increasing the drying costs, and the prolonged drying periods can cause percarbonate decomposition, reducing the value of the eventual product.
It is an object of the present invention to provide a process for stabilising percarbonate particles which ameliorates or overcomes at least some of the disadvantages of the above-mentioned processes and/or provides an alternative to the above-described processes,